This application is directed to an insert system for influencing the pattern of material flow in storage containers for bulk solids.
Bulk storage containers, variously referred to as hoppers, silos, bunkers and bins, are widely used for the temporary storage of quantities of loose solids. For the purposes of this application, the term `hopper` will be used to cover all differing forms of storage containers for loose bulk solids where the material is filled into the top of the container and it moves during the discharge process to an outlet situated in the lower regions of the container.
The manner in which the contents move during the discharging process is essentially characterised by whether all the contents are in motion, termed `Mass Flow`, as shown by the reference numeral 1 in FIG. 1. or whether an internal channel of flow (2) develops within a bed of static material, (3) termed `Funnel Flow` or `Core Flow`, as shown in FIG. 2.
Storage containers are commonly made in the form of a cylindrical body section (4) fitted with a concentric conical converging section, (5) as shown in FIG. 3. Further common shapes are of rectangular or square cross sections, (6) with either a pyramid shaped base section, (7) as shown in FIG. 4, or a construction with a Vee section (8) converging to an outlet slot, (9) as shown in FIG. 5.
The `Mass Flow` form of movement of hopper contents offers various operating advantages, but the converging wall surfaces of the container require to be much steeper than is the case with `Funnel flow` type hoppers. `Mass Flow` hoppers therefore have the disadvantage of requiring greater headroom in order to store a particular volume of product, or of storing less volume within a limited headroom.
Mass Flow hoppers also require specialised design based upon measured properties of the material to be stored. This expertise and bulk material testing procedure tends to be expensive in relation to the manufacturing cost of many hoppers used in the process industries.
As a consequence most hoppers in service are of the `Funnel Flow` type. Many of these hoppers experience problems associated with this form of material flow. Any segregation which takes place during filling is not corrected when the material is discharged. Flow stoppages can occur due to the material `bridging` as a stable mass over the outlet. The discharge may have erratic and/or limited flow rates. The density and behaviour of the product varies when filled into sacks, keg, bins, drums or other containers. `Flushing` i.e. uncontrolled discharge of the product in a fluid state is also a performance hazard. There is always an indeterminate and extended storage periods for some portion of the contents because the order of discharge is not related to the sequence with which the differing regions of the hopper are filled. This feature may lead to deterioration of the products condition, its flowability or other forms of adverse behaviour.
The invention provides a flow deflecting and shielding system, which influences the flow pattern of bulk material in the hopper, such that all the stored contents move in a Mass Flow manner during the discharge process, in hoppers which have shallower wall angles than those normally required.
The angle of wall inclination required to promote Mass Flow of the hopper contents, is a function of the frictional characteristics of the bulk material on the contact surface of the container wall and of the internal angle of friction of the bulk material. The required angle of wall inclination to promote wall slip in containers of cone and wedge shape construction is described in the technical paper `Gravity Flow of Bulk Solids`, published by A. W. Jenike, Bull 126, University of Utah, 1965.
The mechanism by which bulk materials are held in a firm position against the wall of a container, whilst an internal flow channel develops in the body of the stored material during outflow, is the result of a compound assembly of stresses. In the case of a conical hopper these comprise of three components, respectively generated by:
1. Wall Friction--Resistance to wall slip is mobilised by potential movement of the material relative to the wall (10) as shown in FIG. 6 giving rise to an opposing force (11) parallel to the contact surface, (12) because of the friction of the material against the wall. The magnitude of this resisting force is a function of the interface characteristics between the bulk material and the wall surface, and is proportional to the contact pressure (13) which is acting at 90.degree. to the wall surface. The required wall inclination for Mass Flow is closely related to the wall friction angle of the stored solid sliding on the contact surface of the container walls. PA1 2. Radial Pressure--Radially acting pressure from flowing contents in a central `core` region of the hopper contents (14), as shown in FIG. 7 acts against the supporting surface of the static bulk material at the flow boundary interface 15. This pressure not only resists the boundary layers moving radially inwards, but also enhances the ultimate outward pressure against the container wall to result in an increase in the wall friction effect. PA1 3. Circumferential Pressure--Resistance to a reduction in the circumferential dimension of material in an outer annular region of a cone shaped hopper is generated by virtue of the bulk material being subject to a compressive `hoop` stress (16) as shown in FIG. 8 as the material commences to move down within the converging section into a cross section of reduced diameter. The presence of the outer container wall and of material occupying the central region of the cross section provides a state of confinement of the annular bulk, to oppose deformation of this material, and hence its ability to move to the lowers region of the hopper with its smaller cross section.
These components can be considered in detail:
1. Wall Friction.
Changes of the slip characteristics of the bulk material on the wall contact surface influence the hopper geometry required to provide wall slip. Differing surface finishes or materials of construction, wall liner materials and surface coatings are commonly used to improve wall slip. In some eases the condition of the bulk material itself is modified to give better flow characteristics.
This approach has strict limitations, in that the range of suitable materials for construction or lining the wall surfaces are limited by the friction values available, and also by cost and other criteria of use. Surface frictional values are inherent properties of the interface characteristics between the bulk material and the contact surface and lower values may not be achievable. Fixing methods for facing materials may also raise problems of flow, hygiene and the durability of the installed surface.
It is also found that differing materials used for hopper wall construction do not always exhibit similar relationships of frictional values with differing bulk materials. A surface which has a lower value of friction surface than another surface with one bulk material may have a higher frictional value when used with another bulk material, or even with the same material when it has a differing moisture content, temperature or other variant.
There is no ubiquitous `low friction surface`. Measured values of contact friction are needed to establish optimum contact surface materials for specific products.
2. Radial Pressure.
A prior invention for stimulating mass flow in a hopper, UK Patent No. 2,056,296 B, consists of fitting an inner cone to the hopper. FIG. 9. This inner cone (17) has steep walls to stimulate mass flow of the inner contents (18) and its inner walls sustain the pressure acting radially outwards (19) from this centrally flowing region. Material in the outer annulus (20) is therefore able to deform more easily by virtue of containment of the active radial pressures of the central region of flow. This material in the outer regions of the hopper diameter is thus able to flow and slip on the outer walls at lower inclinations than if the inner cone was not fitted. A characteristic of this system is that the inner and outer regions are essentially separate flow channels where the form of flow in each is dictated by their respective geometries and contact conditions. Each section requires its separate extraction conditions (21), (22) to be satisfied.
3. Circumferential Pressure.
An alternative approach, developed earlier by the applicant, FIG. 10, provides an inclined tubular form of insert (23) to shield the outlet region (24) and direct the extractive flow channel from the outlet (25) to behind the insert. (26) Dilation of the flowing media underneath the insert provides a region of reduced pressure into which the remaining cross section of material (27) may flow in order to reduce in diameter as it moves down within the hopper. This allows the whole contents of the hopper to flow in a mass flow manner. The reduction in circumferential stress provided by this insert permits the material in the hopper cross section to deform more easily, and enables slip to develop on the hopper walls at lower inclinations than if it were not fitted.
Drawbacks of the design methods described above are that they are cumbersome and expensive to manufacture and they are relatively difficult to install, particularly when supplied as retrofits to improve flow in existing hoppers. The forms of insert referred to sustain high structural loads, due to their manner of offering support to the flowing contents.
The invention incorporates features to provide both radial and circumferential stress relief for the contents of conical hoppers in the region of the walls and also cause a reduction of the friction forces opposing slip on the walls by reducing the wall contact pressures. For wedge and pyramid shaped hoppers the invention provides support for flow forces acting outward towards the wall and relief for horizontal forces in the flowing media acting parallel to the walls. Deformation and Mass Flow of the product takes place at lower wall inclinations than with hoppers not fitted with these inserts.
The components described in the invention are more simple to manufacture and fit than the previous inserts described, particularly for installation in existing storage hoppers. They also allow more robust and simpler methods of support, by virtue of their basic design.